In recent years, contactless integrated circuits have been developed that operate through an antenna coil, including receiving or transmitting data by inductive coupling in the presence of a magnetic field transmitted by a data transmitting and/or receiving station. These integrated circuits, which are also called passive transponders, can be used to produce various contactless electronic portable objects such as smart cards, electronic tags and electronic tokens, for example.
The present invention concerns the production of such portable objects, and, more particularly, the production of the electronic part of such objects. The most frequently used method to produce the electronic part of a contactless portable object includes using a support wafer on which a coil and a silicon chip are arranged. The coil is then connected to the chip and the unit is covered with protective resin. Generally, the support wafer is a printed circuit board. The coil is a copper wire glued on or an etched copper strip. The coil and chip are connected by ultrasonically bonded metal wires. The assembly forms an electronic micromodule designed to be inserted into the body of a portable object (plastic card, token, land, key . . . ) or fixed onto the surface of such an object.
The inconvenience of this method is that it involves the micromodule components being handled at several steps and requires assembling, wiring and controlling steps which increases the cost of micromodules and restrict rates of output. Moreover, with this method it is not possible to produce very thin micromodules.
Generally, the printed circuit board is approximately 150 micrometers thick, the silicon chip is approximately 150 micrometers thick once the rear side has been chemically or mechanically abraded, and the height of the loops formed by the wiring cables is in the region of 120 micrometers. Finally, the thickness of the resin coating the wires is between 20 and 50 micrometers. In total, the thickness of a classic micromodule is in the region of 400 to 500 micrometers. In comparison, the thickness of a plastic card is approximately 760 micrometers. Contactless smart cards that contain this type of micromodule are often uneven.
Several methods are also known by which it is possible to collectively produce a plurality of coils on one silicon wafer comprising a plurality of integrated circuits, such as the method described in U.S. Pat. No. 4,857,893, for example. After cutting the silicon wafer, very thin integrated micromodules are obtained. The handling, assembly and connection phases of chips and coils are thus removed.
However, the surface area provided by a silicon chip, which is a few square millimeters, is insufficient to produce a high inductance coil. Integrated circuits fitted with an integrated coil are therefore reserved for so-called "proximity" applications. This is where the electromagnetic induction communication distance is short, and is on the order of one millimeter.
Moreover, it is also conceivable to produce larger scale coils on a silicon wafer, such as coils surrounding the areas where the integrated circuits are located, for example. However, this approach is inconvenient in that it reduces the number of integrated circuits that can be produced together on a single silicon wafer, and thus increases the cost. In the semiconductor industry, the cost of a silicon chip is determined by the production cost of the silicon wafer divided by the number of chips produced. Thus, for example, the production of 6 mm.sup.2 coils on a silicon wafer comprising integrated circuits of a surface area of 2 mm.sup.2 increases the cost price of each integrated circuit three-fold.
Finally, the methods that include integrating electronic circuits and coils in one silicon wafer do not seem to be advantageous despite the gain in labor due to the removal of the need to assemble and wire the coils and the integrated circuits. Several technological methods are already known, by which it is possible to produce integrated coils together and at a low cost, particularly the polyimide/silicon dioxide/copper multilayer method on a silicon wafer. Once separated, the coils are in the form of small chips that can be assembled and connected to integrated circuit chips. Nonetheless, the same problems of labor arise due to the need to handle, assemble and connect small individual components two at a time.